Support custom Eloquent pivot accessor names

[shuvroroy]

Summary

Follow-up to #266 based on review feedback about Laravel’s custom pivot accessor names.

Many-to-many relationships using ->as('participation') now expose $participation on the related model instead of $pivot. The accessor retains the configured pivot model type for completion, hover, and chained member access.

This also supports:

• Literal accessor names from BelongsToMany’s fourth generic.
• Pivot types supplied by the third generic or ->using(...).
• Multiple differently named pivot accessors targeting the same model.
• Safe handling of dynamic ->as($name) calls without synthesizing an incorrect property.
• Existing declared properties without overwriting or duplicating them.

Relationship hover information now includes the custom pivot accessor. The Laravel demo and changelog have also been updated.

Performance

• Accessor and related-model names use interned atoms.
• The reverse index uses identity-hashed atom keys.
• Relationship metadata is looked up once per relationship.
• Target classes are cloned only when an accessor actually needs to be injected.
• Accessor parsing avoids temporary string allocations

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, examples/)
• I have updated the config schema (config-schema.json)
• I have added/updated tests to cover my changes

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